1. Filed of the Invention
The present invention relates to a pre-amplifier used in an optical communication system, especially relates to a pre-amplifier having a trans-impedance configuration.
2. Related Prior Art
In a light-receiving system used in the optical communication system, the pre-amplifier that converts an optical signal received by a light-receiving device into a voltage signal is necessary to have a low noise feature and a wide dynamic range. The trans-impedance amplifier is well known to satisfy such requests and is widely used in the optical communication system. Typical trans-impedance amplifier has a configuration that a feedback resistor is connected between the input and the output thereof, the impedance of which may be varied as the input power, thus the wide dynamic range may be obtained. A Japanese patent published as H09-232877 has disclosed such configuration of the trans-impedance amplifier.
FIG. 11 is a schematic figure showing a typical pre-amplifier. The pre-amplifier configures an amplifying circuit 2 with a trans-impedance 3 connected in parallel to the amplifying circuit 2, and a light-receiving device such as photodiode connected in an input terminal of the amplifying circuit 2. The trans-impedance 3 has, for example, a feedback resistance Rf with a variable resistance device Qf, such as field effect transistor (FET), connected in parallel thereto. The impedance of the variable resistor may be varied with a control signal output from the control circuit 4 according to the output of the amplifying circuit 2.
The light-receiving device 1 outputs a current Iin corresponding to the magnitude of the optical signal received thereby, and the current, when the input impedance of the amplifying circuit 2 is large enough, may flow into the trans-impedance 3 and make a potential drop therein, thereby generate the output Vout of the amplifying circuit 2. That is, the signal current Iin may be converted into the voltage signal Vout. Setting the feedback resistance is Rf, the relating between the signal current Iin and the voltage output Vout becomes Vout=□Iin*Rf. In this pre-amplifier, enough sensitivity may be obtained for the faint signal current, namely, the output signal with enough magnitude may be realized, by setting the resistance Rf of the feedback resistor large.
However, when the resistance of the feedback resistor Rf is large, saturation in the output signal may occur at the large signal current, which distorts the waveform of the output signal. Such saturation in the output of the pre-amplifier may be prevented by setting the impedance of the trans-impedance 3 equivalently small when the large signal current is input thereto, thus wide dynamic range may be obtained.
In general, as shown in FIG. 11, a parallel connection of a FET Qf with the feedback resistor Rf is well known for the trans-impedance circuit 3. In this circuit, when the input current is below a preset value, the transistor Qf is switched OFF and only the feedback resistor Rf may operate as the trans-impedance. When the output of the pre-amplifier becomes large and the input current exceeds the preset value, the gate bias of the transistor Qf is varied under the control of the control circuit 4, which makes the conduction between the source and the drain and the trans-impedance 3 small, thereby preventing the saturation in the output of the pre-amplifier.
Further, when the single device receives optical signals each having different transmission speed, the light-receiving sensitivity is widely scattered. One configuration, in which a plurality of resistors each serially connected may be selectively switched ON/OFF, is well known. The United States patent U.S. Pat. No. 5,382,920 has disclosed such configuration. According to the configuration above, the feedback impedance may be widely varied, thereby following the change of the transmission speed.
Still another art has disclosed that, providing a wideband amplifier capable of covering the high transmission speed and a plurality of band-pass filter each connected to the wideband amplifier, and the optical receiver may be operated at different transmission speed by selecting one band-pass filter optimal to the transmission speed practically received by the optical receiver. This technique has been disclosed in Japanese Patent application published as H10-150417. Generally in the amplifier, the product of the gain and the bandwidth thereof (GB product) shows constant. Therefore, using the wideband amplifier, the gain of the amplifier must be reduced, accordingly, restricting the application of the amplifier.
In the recent optical communication, one transmission system is used, in which two standards of the SONET (Synchronous Optical NET) in accordance with the United States standard and the SDH (Synchronous Digital Hierarchy) in accordance with the international standard are multiplexed. Various transmission speeds, for example 155.52 Mbps and its integral multiplications such as 622.08 Mbps and 2.48832 Gbps, are specified commonly to the SONET and the SDH standards. In these cases, the sensitivity specified for transmission speeds are −34 dBm, −28 dBm and −18 dBm, respectively. The higher sensitivity is required for the lower transmission speed.
To obtain a trans-impedance pre-amplifier operating at such widely different transmission speed requires not only the function of receiving optical signals but also the performance satisfying the optical sensitivity specified in the standard for respective transmission speed. One solution is provided in a trans-impedance amplifier shown in FIG. 11, in which the feedback resistance thereof may be precisely adjusted as the transmission speed varies.
On the other hand, the bandwidth of the trans-impedance amplifier, which is denoted as a frequency fc where the gain thereof decreases by 3 dB compared to that at low frequencies, is denoted as follows when the intrinsic gain of the amplifier is a finite value:fc=1/(2π·R·Cs)/(1+1/A),where A is the intrinsic gain of the amplifier, R is the resistance of the feedback resistor, and Cs is input capacitance, which is sum of the junction capacitance of the light-receiving device, input capacitance of the pre-amplifier, and parasitic capacitance derived from the configuration of the assembly of the pre-amplifier and the light-receiving device. The frequency bandwidth of the pre-amplifier may vary by adjusting the resistor R with the change of the transmission speed. However, to obtain an optimal frequency bandwidth of the pre-amplifier, it is insufficient to adjust the resistor R but requires to controlling the intrinsic gain A of the pre-amplifier.
In prior pre-amplifiers mentioned above, the control of the intrinsic gain A concurrently to the control of the feedback resistance R has not been taken into consideration. Even the control of the intrinsic gain A has been mentioned, the control thereof has been independent on that of the feedback resistance R. Therefore, it has not been solved to acquire an optimal gain, an optimal frequency bandwidth and an input dynamic range for respective transmission speeds.
Therefore, one object of the present invention is to provide a pre-amplifier having a trans-impedance type that receives optical signals whose transmission speeds are widely different to each other and provides a wide dynamic range of the input sensitivity for respective transmission speeds with an optimal frequency bandwidth.